The present invention relates to a mirror device for transferring a fine circuit pattern, a mirror adjustment method, an exposure apparatus, an exposure method, a semiconductor device manufacturing method using the exposure apparatus, and the like.
Reduction projection exposure using ultraviolet light has conventionally been performed as a printing (lithography) method of manufacturing a fine semiconductor element such as a semiconductor memory or logic circuit. The minimum size capable of transfer by reduction projection exposure is proportional to the wavelength of light used for transfer and inversely proportional to the numerical aperture of the projection optical system. To transfer a fine circuit pattern, the wavelength of light for use is being shortened. The wavelength of ultraviolet rays for use is becoming shorter to a mercury-vapor lamp i-line (wavelength: 365 nm), a KrF excimer laser beam (wavelength: 248 nm), and an ArF excimer laser beam (wavelength: 193 nm).
However, semiconductor elements rapidly shrink in feature size, and lithography using ultraviolet light reaches its limit in micropatterning of manufacturing a semiconductor element. To efficiently print a very fine circuit pattern with a semiconductor element line width smaller than 0.1 xcexcm, a reduction projection exposure apparatus using extreme ultraviolet light (to be referred to as xe2x80x9cEUV lightxe2x80x9d hereinafter) whose wavelength is as short as about 10 to 15 nm is being developed.
In the EUV light region, a substance greatly absorbs light. The use of a lens optical system which utilizes light refraction used for visible light or ultraviolet light is not practical. An exposure apparatus using EUV light adopts a reflecting optical system. In this case, a reticle as a master is a reflecting reticle on which a pattern to be transferred is formed by a light absorber.
Reflecting optical elements employed in the exposure apparatus using EUV light include a multilayered mirror and a grazing incidence total reflection mirror. The real part of the refractive index is slightly smaller than 1 in the EUV region, and total reflection occurs in the grazing incidence state in which EUV light is incident almost parallel to the plane. In general, grazing incidence at several degrees from an irradiation plane provides a high reflectance of several ten % or more. However, the degree of freedom in optical design is low, and it is difficult to use the total reflection mirror in the projection optical system.
An EUV light mirror used for an incident angle almost close to normal incidence is a multilayer mirror obtained by alternately stacking two types of substances having different optical constants. Molybdenum and silicon are alternately stacked on the surface of a glass substrate polished into a precise figure. The layer thickness is, e.g., 2 nm for the molybdenum layer and about 5 nm for the silicon layer. The number of stacked layers is about 20 pairs. A value as the sum of the thicknesses of layers of two types of substances will be called a film period. In this example, the film period is the molybdenum layer thickness (2 nm)+silicon layer thickness (5 nm)=film period (7 nm).
When EUV light is incident on the multilayer mirror, EUV light having a specific wavelength is reflected. Letting xcex8 be the incident angle of EUV light on the multilayer mirror, xcex be the wavelength of EUV light, and d be the film period, only EUV light with a narrow bandwidth centered on xcex which approximately satisfies Bragg""s equation (1):
2xc3x97dxc3x97sin xcex8=xcexxe2x80x83xe2x80x83(1)
is efficiently reflected. At this time, the bandwidth is about 0.6 to 1 nm.
The reflectance of reflected EUV light is about 0.7 at maximum. Non-reflected EUV light is absorbed in the multilayer film or substrate, and most of the energy is converted into heat. The multilayer mirror exhibits a large optical loss in comparison with a visible light mirror, and the number of mirrors must be minimized. To realize a wide exposure region with a small number of mirrors, a reticle and wafer are simultaneously scanned (scanning exposure) to transfer a pattern in a large area by using only a narrow arcuate region (ring field) spaced apart from the optical axis by a predetermined distance.
A reduction projection exposure apparatus using EUV light in scanning exposure is mainly constituted by an EUV light source, an illumination optical system, a reflecting reticle, a projection optical system, a reticle stage, a wafer stage, an alignment optical system, and a vacuum system.
As th EUV light source, e.g., a laser plasma source is used. A target material in a vacuum vessel 130 is irradiated with a high-intensity pulse laser beam to generate a high-temperature plasma, and EUV light which is emitted by the plasma and has a wavelength of, e.g., about 13 nm is utilized. The target material is a metal thin film, inter gas, droplets, or the like, and is supplied into the vacuum vessel by means such as a gas jet. To increase the average intensity of emitted EUV light, the repetition frequency of the pulse laser is preferably high. The pulse laser is generally operated at a repetition frequency of several kHz.
The illumination optical system is comprised of a plurality of multilayer mirrors or grazing incidence mirrors, an optical integrator, and the like. A collection mirror on the first stage collects EUV light almost isotropically emitted from a laser plasma. The optical integrator uniformly illuminates a mask at a predetermined numerical aperture. An aperture for limiting a region illuminated on the reticle plane to an arcuate figure is formed at a position conjugate to the reticle of the illumination optical system.
The projection optical system includes a plurality of mirrors. A smaller number of mirrors which constitute the projection optical system provide a higher EUV light utilization efficiency, but make aberration correction difficult. The number of mirrors necessary for aberration correction is about four to six. The reflecting surface of the mirror has a convex or concave spherical or aspherical figure. The numerical aperture NA in this case is about 0.1 to 0.2 (the NA is restricted by a numerical restriction aperture 117 shown by FIGS. 1 and 13). The mirror is fabricated by grinding and polishing a substrate made of a material with a high rigidity, high hardness, and small thermal expansion coefficient, such as low-expansion-coefficient glass or silicon carbide, into a predetermined reflecting surface figure, and forming multilayer films of molybdenum and silicon on the reflecting surface. If the incident angle is not constant depending on the position within the mirror surface, the reflectance of a multilayer film with a constant film period increases depending on the position, shifting the wavelength of EUV light, as is apparent from Bragg""s equation. To prevent this, the film period distribution must be set such that EUV light having the same wavelength is efficiently reflected within the mirror surface.
The reticle and wafer stages have systems of scanning these stages in synchronism with each other at a velocity ratio proportional to the reduction magnification. The scanning direction within the reticle or wafer plane is the X-axis, an in-plane direction perpendicular to the scanning direction is the Y-axis, and a direction perpendicular to the reticle or wafer plane is the Z-axis.
A reticle is held by a reticle chuck 116 on the reticle stage. The reticle stage has a driving system of moving the reticle stage along the X-axis at a high speed. The reticle stage also has fine moving systems in the X-axis direction, Y-axis direction, Z-axis direction, and rotational directions around these axes, and can align a reticle. The position and posture of the reticle stage are measured by a laser interferometer, and controlled on the basis of the measurement results.
A wafer is held on the wafer stage by a wafer chuck 120. The wafer stage has a system of moving the wafer stage long the X-axis at a high speed, similar to the reticle stage. The wafer stage also has fine moving systems in the X-axis direction, Y-axis direction, Z-axis direction, and rotational directions around these axes, and can align a wafer. The position and posture of the wafer stage are measured by a laser interferometer, and controlled on the basis of the measurement results.
An alignment detection system 118 measures the positional relationship between the reticle position and the optical axis of the projection optical system, and the positional relationship between the wafer position and the optical axis of the projection optical system. The positions and angles of the reticle and wafer stages are set such that a reticle projection image coincides with a predetermined position on a wafer.
The Z-axis focus position within the wafer plane is measured by a focus position detection system 119, and the position and angle of the wafer stage are controlled. The wafer plane can always keep an imaging position with respect to the projection optical system during exposure.
At the end of one scanning exposure on a wafer, the wafer stage moves step by step in the X and Y directions to the next scanning exposure start position. The reticle and wafer stages are sync-scanned again in the X direction at a velocity ratio proportional to the reduction magnification of the projection optical system. In this way, sync scanning operation is repeated (step and scan) while the reduction projection image of a reticle is formed on a wafer. As a result, the reticle transfer pattern is transferred onto the entire wafer surface.
However, the conventional EUV exposure apparatus suffers from the following problems. As described above, the reflectance of EUV light reflected by the multilayer mirror is about 0.7 at a maximum. Non-reflected light is absorbed in the multilayer film or substrate, and most of the energy is converted into heat. When the mirror or reticle is irradiated with EUV light, non-reflected light is absorbed in an optical element, generating heat.
EUV light as exposure light is strongly absorbed in gas. For example, when EUV light with a wavelength of 13 nm propagates by 1 m through a space filled with air at 10 Pa, about 50% of EUV light is absorbed. To avoid gas absorption, the space through which EUV light propagates must be maintained at a pressure of at least 10xe2x88x921 Pa or less, and desirably 10xe2x88x923 Pa or less.
If molecules containing carbon such as hydrocarbon remain in a space where an optical element irradiated with EUV light exists, carbon is gradually deposited on the surface of the optical element by light irradiation, and absorbs EUV light, decreasing the reflectance. To prevent carbon deposition, the space where the optical element irradiated with EUV light exists must be maintained at a pressure of at least 10xe2x88x924 Pa or less, and desirably 10xe2x88x926 Pa or less.
Under this situation, the EUV optical element cannot be cooled by heat conduction to ambient gas, and must be cooled via a mirror holder or reticle chuck which holds the optical element. For this purpose, the mirror and reticle are fixed to the mirror holder and reticle chuck equipped with cooling means of circulating constant-temperature cooling water. Heat generated by absorption of EUV light is externally dissipated from the optical element, suppressing any temperature rise of the optical element.
The optical element is irradiated with EUV light only during exposure. During the remaining time, e.g., while the wafer stage moves to the next scanning exposure start position, or reticles or wafers are replaced, the optical element is not irradiated with light, and no energy absorption occurs. In other words, energy to the optical element is not temporarily constant and greatly varies with time. It is, therefore, difficult to keep the optical element at a constant temperature by a cooling method of, e.g., circulating constant-temperature cooling water through the mirror holder or reticle chuck which holds the optical element without considering any temporary change of the heat amount absorbed by the optical element. The temperature of the optical element becomes different between the start of exposure and the progress of exposure, and the figure of the reflecting surface inevitably changes.
The figure is not always kept unchanged even at a constant temperature on the optical element surface due to the temperature distribution inside the optical element and the temperature difference between upper and lower surfaces. Considering deformation factors such as deflection by weight other than the temperature, the optical element figure, i.e., optical characteristic is not always maintained even at a constant temperature on the optical element surface.
The surface figure of the reflecting surface of the projection optical system must have a very high precision. Letting n be the number of mirrors which constitute the projection optical system, and xcex be the wavelength of EUV light, an allowable figure error "sgr" (rms value) is given by Marechal""s equation:
"sgr"=xcex/(28xc3x97√{square root over (n)})xe2x80x83xe2x80x83(2)
For example, for a system having four mirrors and a wavelength of 13 nm, the allowable figure error is "sgr"=0.23 nm.
The temperature rise of an optical element which constitutes the projection optical system causes a surface figure disturbance which exceeds the allowable error. The imaging performance of the projection optical system cannot be fully attained, and the resolution and contrast decrease, failing to transfer a fine pattern.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide an exposure apparatus capable of stably transferring a fine pattern without any decrease in resolution or contrast upon a change in reflecting surface figure caused by temperature variations of an optical element, a mirror device suitable for the exposure apparatus, and the like.
More specifically, the present invention provides a mirror device which constitutes an optical system of an exposure apparatus for transferring a reticle pattern onto a wafer, comprising control means for controlling a reflecting surface figure.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.